2396_AWL_Bennett_Ch17 6/25/03 3:46 PM Page 544 17 Star Stuff LEARNING GOALS 17.1 Lives in the Balance • What prevents carbon from fusing to heavier • What kind of pressure opposes the inward pull elements in low-mass stars? of gravity during most of a star’s life? 17.4 Life as a High-Mass Star • What basic stellar property determines how a star • In what ways do high-mass stars differ from low- will live and die? Why? mass stars? • How do we categorize stars by mass? • How do high-mass stars produce elements heavier 17.2 Star Birth than carbon? • Where are stars born? • What causes a supernova? • What is a protostar? • Do supernovae explode near Earth? • What are the “prebirth” stages of a star’s life? 17.5 The Lives of Close Binary Stars • What is a brown dwarf ? • Why are the life stories of close binary stars different 17.3 Life as a Low-Mass Star from those of single, isolated stars? • What are the major phases in the life of a • What is the Algol paradox? low-mass star? • How did past red giant stars contribute to the existence of life on Earth? 544 2396_AWL_Bennett_Ch17 6/25/03 3:46 PM Page 545 I can hear the sizzle of newborn stars, A star can maintain its internal thermal pressure only and know anything of meaning, of the if it continually generates new thermal energy to replace fierce magic emerging here. I am witness the energy it radiates into space. This energy can come from to flexible eternity, the evolving past, two sources: nuclear fusion of light elements into heavier and know I will live forever, as dust or ones and the process of gravitational contraction, which breath in the face of stars, in the converts gravitational potential energy into thermal energy shifting pattern of winds. [Section 15.1]. These energy-production processes operate only tem- Joy Harjo, Secrets from the Center of the World porarily, although in this case “temporarily” means mil- lions or billions of years. In contrast, gravity acts eternally. Moreover, any time gravity succeeds in shrinking a star’s e inhale oxygen with every breath. core, the strength of gravity grows. (The force of gravity Iron-bearing hemoglobin carries this inside an object grows stronger if it either gains mass or shrinks in radius [Section 5.3].) Because a star cannot gen- oxygen through the bloodstream. W erate thermal energy forever, its ultimate fate depends on Chains of carbon and nitrogen form the backbone whether something other than thermal pressure manages of the proteins, fats, and carbohydrates in our cells. to halt the unceasing crush of gravity. The final outcome of a star’s struggle between gravity Calcium strengthens our bones, while sodium and and pressure depends almost entirely on its birth mass. potassium ions moderate communications of the All stars are born from spinning clumps of gas, but new- nervous system. What does all this biology have to born stars can have masses ranging from less than 10% of the mass of our Sun to about 100 times that of our Sun. do with astronomy? The profound answer, recognized The most massive stars live fast and die young, proceeding only in the second half of the twentieth century, is from birth to explosive death in just a few million years. that life is based on elements created by stars. The lowest-mass stars, in contrast, consume hydrogen so slowly that they will continue to shine until the universe We’ve already discussed in general terms how is many times older than it is today. the elements in our bodies came to exist. Hydrogen Because of the wide range of stellar masses, we can and helium were produced in the Big Bang, and heav- simplify our discussion of stellar lives by dividing stars into three basic groups: ier elements were created later by stars and scat- ● tered into space by stellar explosions. There, in the Low-mass stars are stars born with less than about two times the mass of our Sun, or less than 2 solar spaces between the stars, these elements mixed with masses (2MSun) of material. interstellar gas and became incorporated into sub- ● Intermediate-mass stars have birth weights between sequent generations of stars. about 2 and 8 solar masses. In this chapter, we will discuss the origins of ● High-mass stars are those stars born with masses the elements in greater detail by delving into the lives greater than about 8 solar masses. of stars. As you read, keep in mind that no matter Both low-mass and intermediate-mass stars swell into how far removed the stars may seem from our every- red giants near the ends of their lives and ultimately be- come white dwarfs. High-mass stars also become red and day lives, they actually are connected to us in the large in their latter days, but their lives end much more most intimate way possible: Without the births, lives, violently. and deaths of stars, none of us would be here. We will focus most of our discussion in this chapter on the dramatic differences between the lives of low- and ypla om ce n . o c r o t m s high-mass stars. Because the life stages of intermediate- a Stellar Evolution Tutorial, Lesson 1 mass stars are quite similar to the corresponding stages of high-mass stars until the very ends of their lives, we include 17.1 Lives in the Balance them in our discussion of high-mass stars. The story of a star’s life is in many ways the story of an Given the brevity of human history compared to the extended battle between two opposing forces: gravity and life of any star, you might wonder how we can know so much pressure. The most common type of pressure in stars is about stellar life cycles. As with any scientific inquiry, we thermal pressure—the familiar type of pressure that keeps study stellar lives by comparing theory and observation. On a balloon inflated and that increases when the tempera- the theoretical side, we use mathematical models based on ture or thermal energy increases. the known laws of physics to predict the life cycles of stars. chapter 17 • Star Stuff 545 2396_AWL_Bennett_Ch17 6/25/03 3:46 PM Page 546 On the observational side, we study stars of different mass itself. The clouds that form stars tend to be quite cold, typ- but the same age by looking in star clusters whose ages we ically only 10–30 K. (Recall that 0 K is absolute zero, and have determined by main-sequence turnoff [Section 16.6]. temperatures on Earth are around 300 K.) They also tend Occasionally, we even catch a star in its death throes. Theo- to be quite dense compared to the rest of the gas between retical predictions of the life cycles of stars agree quite well the stars, although they would qualify as a superb vacuum with these observations. by earthly standards. Like the galaxy as a whole, star-forming In the remainder of this chapter, we will examine in clouds are made almost entirely of hydrogen and helium. detail our modern understanding of the life stories of stars Star-forming clouds are sometimes called molecular and how they manufacture the variety of elements—the clouds,because their low temperatures allow hydrogen star stuff—that make our lives possible. atoms to pair up to form hydrogen molecules (H2). The relatively rare atoms of elements heavier than helium can also form molecules, such as carbon monoxide or water, or tiny, solid grains of dust. More important, the cold temper- atures and high densities allow gravity to overcome thermal 17.2 Star Birth pressure more readily in molecular clouds than elsewhere Stars are born from clouds of interstellar gas (Figure 17.1) in interstellar space. If the thermal pressure in a molecular and return much of that gas to interstellar space when cloud is too weak to counteract the compressing force of they die. In Chapter 19, we will examine this star–gas–star gravity, then the cloud must undergo gravitational contrac- cycle in more detail. Here we will focus on star formation tion. Because molecular clouds are generally lumpy, gravity Figure 17.1 A star-forming cloud of molecular hydrogen gas in the constellation Scorpius extends from VIS the upper-right corner of this photo through the center. The cloud appears dark because dust particles within it obscure the light radiated from more distant stars lying behind it. Blue-white blotches near the edges of the dark cloud are newly formed stars. They appear fuzzy because some of their light is reflecting off patchy gas in their vicinity. The region pictured here is about 50 light-years across. 546 part V•Stellar Alchemy 2396_AWL_Bennett_Ch17 6/25/03 3:46 PM Page 547 Figure 17.2 Fragmentation of a molecular cloud. Gravity attracts matter to the densest regions of a molecular cloud. If gravity can overcome thermal pressure in these dense regions, they collapse to form even denser knots of gaseous matter known as molecular cloud cores. The cloud thus fragments into a number of pieces, each of which will form one or more new stars. pulls the molecular gas toward the densest lumps, known as molecular cloud cores. A cloud thus fragments into numer- ous pieces, each of which will form one or more new stars (Figure 17.2). From Cloud to Protostar Betelgeuse Gravitational contraction within each shrinking fragment of a molecular cloud releases thermal energy.